1. Field of the Invention
The present invention relates to a sampling circuit, a phase reference detecting circuit and a sampling clock shifting circuit. A phase reference detecting circuit and a sampling clock shifting circuit are used in a sampling circuit. A sampling circuit is used to reproduce and digitalize a color image signal, for example.
2. Description of the Background Art
FIG. 26 is a circuitry diagram showing a structure of a conventional sampling circuit. Resistors 20a, 20b and 20c are connected to an NPN transistor 21a. The operating bias of the NPN transistor 21a is determined by these resistors and power sources V.sub.CC and V.sub.EE. An NPN transistor 21b is also connected to the NPN transistor 21a through a capacitor 19b and a pre-set resistor 22.
A sampling clock received at an input terminal IN is a sinusoidal wave. The capacitor 19a allows only an ac component to be transmitted to a base of the transistor 21a which is biased to a voltage which corresponds to a voltage ratio of the resistors 20a and 20b. From an emitter and a collector of the transistor 21a, two different signals which are shifted by 180.degree. from each other are outputted. One of these signals is supplied to the capacitor 19b and the other is supplied to the pre-set resistor 22. These signals are then synthesized and supplied to a base of the transistor 21b. A synthesized signal has a phase lag which corresponds to a time constant which is defined as a product of a capacitance value of the capacitor 19b and a resistance of the pre-set resistor 22. The transistor 21b and a resistor 20d form an emitter follower circuit which converts an impedance of the synthesized signal. A resultant signal is outputted at an output terminal OUT.
In short, a phase difference of the sampling clock against a signal-to-be-sampled is adjusted by means of the time constant of the resonance circuit which is formed by the capacitor 19b and the pre-set resistor 22. As a result of adjustment of the phase difference of the sampling clock, sampling points are adjusted.
Having such a structure as above, the conventional sampling circuit is susceptible to an influence of the structural components such as the resistors, the capacitor and the transistors, as well as an environmental temperature, a fluctuation in a power source voltage and other factors. To a further disadvantage, it is impossible to automatize adjustment since feedback control is impossible.